The invention relates to a heat-hardenable binder mixture based on organic resins.
Many and varied chemical reactions have been proposed, and also used, to harden, by cross-linking, the binders in finish coatings. The chemical bonds formed during the cross-linking reaction frequently do not satisfy all the requirements placed on the finish coatings. For instance, ester bonds are sensitive to hydrolysis, and amine groups form hydrophilic imperfections in the baked film.
In the two-component systems one of the reactive groups must frequently be blocked to prevent premature reaction.
In recent years, electrocoating has become widely established for the priming of electrically conductive substrates. The emphasis has hitherto been an anodic electropriming. The resin binders used for this purpose belong to the resins containing carboxyl groups, for example to the maleiate oils, maleiated epoxide resins, alkyd resins, acrylic resins and, in particular, to the maleiated polybutadiens. These resins were rendered soluble in water by salt formation, chiefly with amines, and deposited by the current at the anode in the electrocoating bath. However, the anodic electropriming process contains serious disadvantages. For instance, oxygen is evolved at the anode during the electrical deposition and can modify the resins depositing at the anode in a serious, unfavourable manner. Furthermore, metal ions enter into solution at the anode and are contained in the baked film as imperfections. The metal ions can lead to discoloration and spots. They cause qualitative disadvantages in particular by salt formation and hence by reducing the resistance to water and the anticorrosion protection.
The cathodic electropriming process developed in recent years to commercial maturity is increasingly displacing the anodic process, since the defects described above are largely avoided. For instance, hydrogen, which does not affect the resin binder, is formed during the deposition step at the cathode, where the finish film is now deposited. Since the cathodic deposition can take place in the approximately neutral pH range, there are hardly any metal ions going into solution. The binders suitable for cathodic deposition contain predominantly amino groups which are neutralized with acids to obtain solubility in water.
However, a disadvantage is the fact that hydrophilic amino groups remain in the baked film and are responsible for a reduction of the anti-corrosion protection. Although the reaction of the amino groups with blocked isocyanates in the cross-linking converted the amino groups into the less hydrophilic urethane group, blocking agents, such as, for example, phenol or ketoximes, are liberated at the same time.
Depending on the type of structure of the amino group necessarily contained in the resin, these groups can also be thermally eliminated by .beta.-elimination (German Offenlegungsschrift No. 2,363,074, and German Offenlegungsschrift No. 2,753,861). The resulting contamination of the off-air by large amounts of eliminated protective groups of the blocked isocyanates and amines must also be considered a disadvantage of this process.